u/Smooth-Mushroom-1411

I’m looking for the name of a book.

all I remember is there is a kid and she’s spray painting outside of an art gallery I think. An old man sees her work and is impressed, then she tags along with him. I think she has a favorite artist , and I think her or the artists go to is like turtles or something. the old man is going to see a friend and doesn’t want the kid following him but the orphan does anyway. I think it’s an orphan girl but could be boy. don’t remember how they have a painting but at one point they board a train with it and accidentally leave it on there. I think a nice lady with kids recovers it. At the end they make it to the old man’s friend’s house.

I know that’s not a lot to go on but any help would be amazing!